Conventionally, various vehicle door driving apparatuses have been proposed. For example, a vehicle door driving apparatus disclosed in Patent Document 1 includes a vehicle door movably supported by a guide rail (3), which is fixed to a vehicle body, a drive mechanism (6), which is fixed to the vehicle door, and a cable (7), which is selectively wound and fed out by the drive mechanism. The ends of the cable are respectively connected to the vehicle body at the front end and the rear end of the guide rail. Therefore, when the drive mechanism selectively winds and feeds out the cable, the vehicle door is selectively opened and closed.
Patent Document 2 discloses a drive mechanism that can be applied to such a vehicle door driving apparatus. The drive mechanism includes a planet gear mechanism as a speed reducing mechanism. The planet gear mechanism reduces the rotational speed of a motor and transmits the rotation to a drum by a speed reduction ratio in which a sun gear directly connected to the rotary shaft of the motor serves as a drive shaft. A ring gear coupled to the drum to rotate integrally with the drum serves as a driven shaft, and a carrier serves as a fixed shaft. The speed reducing mechanism is coaxial with the rotary shaft of the motor, and the projected area of the drive mechanism in the axial direction as a whole is reduced.
Patent Document 3 discloses another drive mechanism that can be applied to such a vehicle door driving apparatus. The drive mechanism includes a speed reducing mechanism, including a worm gear having a high speed reduction ratio, and an electromagnetic clutch, which connects and disconnects transmission of the rotation of the motor to the drum via the speed reducing mechanism. The outer diameter of the drum is set in accordance with the relationship between the output property of the motor in relation to the winding speed and the required torque for winding. The outer diameter of the electromagnetic clutch is set smaller than the outer diameter of the drum. In this case, since the outer diameter of the electromagnetic clutch is reduced, the size of the electromagnetic clutch in the axial direction is increased. However, the electromagnetic clutch is stored in the drum to efficiently use the space occupied by the drum. Thus, the size of the drive mechanism is prevented from being increased.